1. Field of the Invention
The present invention relates to semiconductor laser devices for use in optical pickup apparatuses utilizing laser light, and more particularly to a semiconductor laser device used in an optical pickup apparatus for CD (Compact Disc), CD-ROM (Read Only Memory), MD (Memory Disc), CD-R (Readable CD) or the like.
2. Description of the Background Art
Information is read out of an information recording medium like a CD by means of a mechanism as shown in FIG. 4A. Laser light emitted from a semiconductor laser chip 122 is diffracted by a tracking beam generating diffraction grating 123 into a plurality of beams. The beams thus generated pass through an optical-path refracting diffraction grating (signal light diffraction grating) 124 from behind, and are collimated by a collimator lens 125. An objective 126 focuses the parallel beams on a disc 127 having digital information written thereon, which disc reflects the beams. The reflected beams including the information written on disc 127 therein follow the reverse course from objective 126 to collimator lens 125. They are then diffracted by signal light diffraction grating 124, and received by photodiodes D1-D5 constituting a light receiving portion 121. As shown in FIG. 4B, tracking beam generating diffraction grating 123 and signal light diffraction grating 124 are normally provided on lower and upper surfaces, respectively, of a hologram 120 being a transparent member. Signal light diffraction grating 124 is formed of two gratings: a fine grating 124a having diffraction grooves of smaller pitches and a coarse grating 124b having diffraction grooves of larger pitches.
Light receiving portion 121 is formed of a plurality of photodiodes D1, D2, D3, D4 and D5, as shown in FIGS. 5A, 5B and 5C. A beam is adjusted such that it is focused right on a signal track of a CD or the like and that the reflected beam falls on a focused point where it achieves focus on the light receiving portion. Herein, deviation of the beam spot from this focused point is called a focus error. FIG. 5A shows a case where the focus error occurs as the light receiving portion is too far from hologram 120, so that signal strength Id2 at photodiode D2 is too strong. FIG. 5C shows a case where the focus error occurs because the light receiving portion is too close to hologram 120, so that signal strength Id3 at photodiode D3 is too strong. These photodiodes D2 and D3 constitute a pair of photodiodes for use in detecting a focus error signal, separated from each other by a parting portion interposed therebetween. FIG. 5B shows a focused state with no focus error, in which case the beam spot is formed right on the parting portion between focus error signal detecting photodiodes D2 and D3. Although this parting portion may be a zonal boundary having a prescribed width, it is herein called a parting line 105 for convenience. When the signal strength (I) at each photodiode is represented with the corresponding reference character added thereto, signal strengths of regenerative signal RF, focus error signal FES and tracking signal RES are expressed as follows:RF=Id2+Id3+Id4FES=Id2−Id3RES=Id1−Id5.
The reflected beam does not always form a spot on the focused point, due to a temperature change or other reasons. The focus error will be prevented if the beam spot moves, during the detection of the focus error signal, on light receiving portion 121 within the region of parting line 105. Since the focus error signal is obtained by Id2−Id3 as described above, it remains zero as long as the beam spot moves within the region of parting line 105 between photodiodes D2 and D3, and no finite focus error signal will be generated. If there is a temperature change, however, the beam spot 111c would move across photodiode D2 or D3, as shown in FIG. 6B, in which case the focus error signal would have a substantial absolute value. In order to suppress such a focus error due to the temperature change, it is necessary to maintain the focus error signal at zero despite the displacement of beam spot 111c. This can be done if the light receiving portion is arranged such that the beam spot moves within the parting line region 105 in accordance with the temperature change. In the case where beam 111 is deflected to fall on either one of focus error signal detecting photodiodes D2 and D3, the focus error signal FES takes a finite value, since FES=Id2−Id3, so that the focus error is detected.
A major factor causing such a focus error is a temperature change. In general, wavelength of the laser light emitted from a semiconductor laser chip changes in accordance with the temperature change. More specifically, the wavelength becomes longer as the temperature increases, and it becomes shorter as the temperature decreases. The configuration shown in FIG. 4A includes two diffraction gratings: tracking beam generating diffraction grating 123 and signal light diffraction grating 124. If the diffraction angle is represented as θ, the wavelength λ and the diffraction grating pitch d, then sin θ=λ/d in these diffraction gratings. It means that, as the wavelength λ changes, the diffraction angle θ changes correspondingly. Even a slight change in temperature would alter the diffraction angle of the laser beam, which causes the beam spot to diverge on the light receiving portion, resulting in a focus error. A laser beam emitted from semiconductor laser chip 122 is diffracted twice before reaching light receiving portion 121, by tracking beam generating diffraction grating 123 and signal light diffraction grating 124, and suffers the focus error due to the temperature change at each diffraction. Normally, tracking beam generating diffraction grating 123 and signal light diffraction grating 124 are arranged such that their directions of diffraction are orthogonal to each other.
FIG. 6A is a top plan view showing a two-dimensional arrangement of conventional light receiving portion 121 and hologram 120 of the optical pickup apparatus shown in FIG. 4A. The beams are aligned such that respective beam spots 111a-111f are formed approximately at the center portions of corresponding photodiodes, except that beam spot 111c is formed on parting line 105 separating focus error signal detecting photodiodes D2 and D3 from each other. Conventionally, parting line 105 has been arranged with a slope, on the order of 10 to 20 mrad, directed from the fine grating 124a side to the coarse grating 124b side as it becomes farther from hologram 120. Photodiode D4 has been arranged with a slope such that its long side extends in an opposite direction from that of the parting line, i.e., from the coarse grating 124b side toward the fine grating 124a side as the distance from hologram 120 increases. FIG. 7 shows measurements of the focus errors with respect to the temperature changes with this arrangement of FIG. 6A. It shows that the amount of the focus error increases in accordance with the temperature increase.
The arrows in FIG. 6B show the directions in which respective beam spots would move in accordance with the temperature change. Conventionally, the light receiving portion itself has been arranged at a certain angle, as shown in FIG. 6C, to make the beam 111c move along parting line 105 when a temperature change occurs. This arrangement can restrict the focus error due to the temperature change.
With this arrangement of the light receiving portion in FIG. 6C, however, some of the beam spots contributing to tracking error signal (RES) and others would deflect from the corresponding photodiodes due to the temperature change. For example, two beam spots 111a and 111e will not fall on the corresponding photodiodes D1 and D5, respectively, thereby weakening the relevant signal. As such, there has been a strong demand for development of a semiconductor device and a pickup apparatus that can restrict a focus error due to a temperature change and prevent reduction in strength of respective signals.